As known processes of producing organic acids by conversion from their corresponding nitriles, mention may be made of chemical synthetic means and biological means. The latter involves the use of a microorganism or a microorganism-derived enzyme as a catalyst to hydrolyze nitriles, so this means is advantageous in that organic acids can be produced under mild conditions. Microorganisms belonging to the genus Rhodococcus are known as such catalysts for use in production of amides or organic acids by hydration or hydrolysis of their corresponding nitriles (see Japanese Laid-Open Patent Publication Nos. 251,192/1991, 91,189/1987, 470/1990, and 84,198/1990).
As compared with the above-mentioned conventional processes, the use of a nitrilase gene cloned for hydrolysis of nitriles by genetic recombination is expected to drastically improve the catalytic ability of the microorganism to hydrate nitriles because the microorganism can be engineered to contain multiple copies of the same gene. To obtain such a catalyst organism with higher catalytic activity, the present inventors successfully cloned a nitrilase gene from the strain Rhodococcus erythropolis SK92 and constructed a plasmid by inserting said gene into a region downstream of an E. coli lactose promoter. By introducing this plasmid into E. coli, the organism came to exhibit higher nitrilase activity during incubation in the presence of IPTG isopropyl-.beta.-D-thiogalactoside). The present inventors further attempted to obtain a transformant of the genus Rhodococcus to attain higher performance as a catalyst organism. In this attempt, the nitrilase gene was inserted into a Rhodococcus-E. coli hybrid plasmid vector (see Japanese Laid-Open Patent Publication Nos. 4,589/1993 and 68,566/1993), and the vector thus constructed was introduced into a microorganism of the genus Rhodococcus. However, no nitrilase activity was expressed, and there is demand for a method of permitting the expression of nitrilase activity in a transformant of the genus Rhodococcus.